Plasma membrane (PM) receptors of mammalian cells have been found to mediate the binding and endocytosis of a variety of extracellular macromolecules. However, little direct information is available regarding their mechanisms of action or their metabolic fate since very few receptors have been isolated and characterized. Among these is the trypsin-resistant Fc receptor of murine macrophages. In order to provide a model for the analysis of adsorptive endocytosis, I intend to study the uptake of IgG by the macrophage Fc receptor (FcR) using an anti-FcR monoclonal antibody as a specific probe. Novel radiolabelling techniques which permit the selective iodination of internalized membrane (and which I developed) will be employed to analyze directly the fate of the FcR during endocytosis. In addition, a set of other monoclonal antibodies directed against unrelated macrophage PM proteins will be utilized to help assess whether ligand binding selectively alters receptor internalization, degradation, and/or biosynthesis. The possibility that the FcR and other membrane proteins are recycled to the cell surface following the receptor-mediated uptake of IgG will be addressed directly using immune precipitation and EM-autoradiography. The role of coated vesicles in the endocytosis of the FcR, both in the presence and absence of bound IgG, will be examined. I will also continue my efforts to characterize the FcR biochemically by 1) generating new anti-FcR antibodies directed against the isolated protein (to identify its different functional domains) and 2) using these antibodies, as well as peptide mapping, specific radiolabelling, and chemical cross-linking, to define the disposition and organization of the FcR in the membrane. Significantly, I intend to use this structural information to determine whether IgG binding induces alteraticns either in the FcR itself or in its interactions with other cellular components.